"[This is] a very important and absolutely horrifying report," said primatologist Frans de Waal, director of the Living Links Center at Emory University.

"There have been isolated pieces of data around for years, which have sketched an ever darker picture," de Waal said, adding that the report supports the bleaker prognoses.

Hundreds of international experts helped to classify 634 primates for the Red List of Threatened Species using criteria established by the International Union for the Conservation of Nature (IUCN). They found nearly half the species and supspecies are endangered.

Scientists have discovered 53 new primate species since 2000, including 40 on Madagascar alone, and no one knows what others may exist. Some could vanish before they are even known to science.

The report, released at the 22nd International Primatological Society Congress in Edinburgh, Scotland, was funded by Conservation International (CI), IUCN, the Margot Marsh Biodiversity Foundation, and Disney's Animal Kingdom.

"Scary" Situation in Asia

The news was particularly bad for Asian primates—more than 70 percent of which are listed as "vulnerable," "endangered," or "critically endangered."

"I think what's most alarming is just how bad the situation is in the Asian region, particularly in Southeast Asia," said Mike Hoffmann, an IUCN scientist based in Washington, D.C.

"In countries like Vietnam and Cambodia pretty much 90 percent of the primate fauna [including gibbons, monkeys, and langurs] is at risk of extinction. That is pretty scary."